The present invention relates to charging of batteries and more particularly to maintenance charging of lithium-based battery cells such as lithium-ion and lithium-polymer battery cells.
Portable electrical devices such as mobile phones, laptop computers, video cameras and others require one or more battery cells to supply electrical power to the device. It is well known that battery cells have a limited charge period and must periodically be connected to an external charger to be recharged. The external charger may include logic instructions for recharging the battery cell in a safe and efficient manner. Alternatively, the electronic device may include logic instructions for controlling the battery cell charging process so that the battery cell may be recharged by connecting the entire electronic device, with the battery intact, to a suitable power source. The logic instructions may be implemented in software operational on a general purpose processor or may reside on an application specific integrated circuit (ASIC).
Several types of battery cells are presently available for portable electronic devices, including nickel-cadmium (Ni--Cd), nickel-metal-hydride (NiMH), and lithium-ion (Li-ion). Li-ion is a relatively new technology that typically uses a lithium-metal-oxide compound as the positive electrode (e.g., cathode) and a carbon-based negative electrode (e.g., anode). Battery charging and discharging occur through the migration of lithium ions between the cathode and anode and the exchange of electrons through doping and de-doping. These electrons represent an electrical current which may be used to power an electrical device. Li-ion battery systems have many advantages over nickel-based battery systems. For example, Li-ion battery systems have a much higher energy density by weight. Accordingly, Li-ion battery systems may be much lighter and longer lasting than nickel-based battery systems. Additionally, in Li-ion battery systems there is no cell-memory effect (i.e., the tendency of a battery cell to resist charging beyond the charge level to which the battery was most recently charged.)
Most Li-ion battery cell chargers use a two-phase charging process in which the charger works first with a constant current, then finishes charging at a constant voltage. This process is referred to as "constant-current, constant-voltage" ("CC-CV") charging. In the first phase of the charging process, a constant current is applied to the Li-ion battery until the cell approaches its maximum voltage. In the second phase, a constant voltage equal to the fully-charged cell voltage is applied to the battery until the battery is fully charged. Li-ion batteries commonly have a useful life that measures between 200 and 1000 charge cycles. Each time the battery is fully charged to its maximum voltage, the useful life of the battery is reduced. Thus, there is a need in the art for a system and method for charging a Li-ion battery cell that maintains a high percentage of the capacity of the battery cell without fully charging the battery which, in turn, reduces its useful life. The present invention utilizes novel charging techniques to address this need.
Additionally, the use of lithium-based battery cells in mobile communication devices, including mobile phones, creates some special design considerations. Foremost, market demands require that mobile phones be extremely physically compact. Additionally, many users are not willing to pay the additional cost required to build intelligence into battery chargers. Presently, many mobile phones include an ASIC that regulates the current and voltage for charging a battery connected to the phone. Thus, the phone's battery may be recharged by connecting the entire mobile phone, with the battery intact, to a "dumb" current source, and logic instructions in the phone's ASIC control the charging process. When the battery is fully charged, a switch in the phone disconnects the phone from the power source to terminate the charging process. The switch required by this design consumes valuable space in the mobile phone.
Accordingly, there is a further need in the art for systems and methods for charging Li-ion batteries that do not require intelligent chargers and are highly space-efficient. The present invention utilizes novel battery charging systems and techniques to enable the maintenance charging of lithium-based battery cells for use with mobile phones.